We describe a female renal transplant recipient with cytomegalovirus and Epstein-Barr virus (EBV) infections who developed aggressive polymorphous polyclonal B cell proliferation. She received two courses of OKT3. We found a majority of transformed B cells bearing EBV membrane receptor CR2 and EBV nuclear antigen. Posttransplant lymphoproliferative disorders may be associated with a significant immunological activation, detected in this case by the sudden increase of beta 2-microglobulin and immunoglobulin levels, including immunoglobulin D. These raised levels persisted throughout the short and rapid course of the disease.